The basic principles of switch-mode power supplies connected in parallel are described, for example, in Elektronik, Volume 13, 2000, pages 114–118 “Schaltnetzteile parallel geschaltet-technische Details zur passiven Stromaufteilung” by Martin Rosenbaum. The aim of connecting switch-mode power supplies in parallel is to increase the power by increasing the output current and to reduce the failure rate by providing redundant switch-mode power supplies in such a way that a defective power supply unit can be exchanged during the operation of a device which is supplied by the switch-mode power supplies. Connection in parallel can be realized by means of an active current division or a passive current division.
Active current division measures the output current of each power supply and controls the output voltages as a function of the output current of all switch-mode power supplies resulting in a uniform division of current to one or more loads. This method has the advantage that an exact division of current and a uniform load on the switch-mode power supplies connected in parallel can be achieved. The disadvantages can be seen in the greater complexity of the circuitry and the higher costs thus incurred.
In the case of passive current division, the current division is made as uniform as possible by setting a “softer output characteristic” for the switch-mode power supply as shown, for example, in FIG. 1. The advantages include a less complex circuitry and the almost limitless number of switch-mode power supplies that can be connected in parallel. A disadvantage is the somewhat less exact division of current in some applications.
FIG. 2 shows a block diagram of an example for N switch-mode power supplies 10, 11, 12 connected in parallel which supply a load 13. Further details on the circuit illustrated in FIG. 2 are illustrated and explained in the above-mentioned article in Elektronik 13/2000. Reference is made to this publication.
In order to set the output characteristics of the individual switch-mode power supplies connected in parallel as required, where passive current division is concerned the prior art provides for one or more shunt resistors to be added to the output line of the respective switch-mode power supplies so that the output voltage of the respective switch-mode power supplies is established as a function of the load, within certain tolerances, according to predetermined characteristics. FIG. 3 shows an example of such an output characteristic for a single switch-mode power supply which has three ranges that can be established by the provision of three shunt resistors connected in the output line of the switch-mode power supply.
The output characteristic shown in FIG. 3 occurs in a first range I, which characterizes the normal operation of the switch-mode power supply, being relatively flat with only a slight voltage drop following an increase in output load and thus an increase in output current. A first shunt resistor RVS is active in this range I, which could also be formed by the line resistance. When the output current I0 exceeds a first threshold value I0P, a second shunt resistor RP is connected which causes the voltage at the output of the switch-mode power supply to fall more strongly. This second range, indicated by II, can, for example, be a charging range in which the power supplies not only supply a load but also charge batteries or other energy storage units, which are intended as an emergency power supply to supply the load during a power failure.
When the output current I0 of the respective switch-mode power supply exceeds a further threshold value I0S, a third shunt resistor is activated which is dimensioned in such a way that the voltage output characteristic of the switch-mode power supply declines abruptly. This range, indicated by III, can be considered a safety cut-off range in which the switch-mode power supply is short circuited and turned off when a specific threshold current I0S is exceeded. The third shunt resistor is indicated by RS.
Although the above-described prior art solution for establishing the output characteristic of the switch-mode power supplies as a function of the output current has a simple circuitry and allows for different operating ranges of the output characteristic, the shunt resistors, which can be located in the power supply unit or outside it in the output line of the switch-mode power supply (s.RL in FIG. 2), generate considerable losses and thus reduce the overall efficiency of the power supply and of the system.
The object of the invention is to provide a power supply device having several switch-mode power supplies connected in parallel to supply at least one load which operates with a passive current division and allows the output characteristic of each switch-mode power supply to be adjusted for different operating ranges. This object is solved by a power supply device having the characteristics outlined in claim 1.